Mechanical safety cover interlock for injection molding machine

ABSTRACT

A mechanical interlock mechanism for the safety cover of the die closing unit of an injection molding machine which has an arresting rod attached to the movable die carrier frame and extending rearwardly through a stationary cylinder head plate, where an arresting unit encloses a pair of arresting jaws with attached cam followers. The latter are engaged by a control rod which is connected to the safety cover by means of a control linkage. The jaws are forcibly closed, in opposition to jaw opening springs, and butting of the arresting jaw ridges against the arresting rod collars is limited to one jaw only, as a result of an axial offset between the arresting jaw ridges, in connection with a certain minimal pitch of the arresting rod collars and a floating arrangement of the jaw closing assembly. The latter is spring-biased towards one side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to injection molding machines and, moreparticularly, to a mechanical interlock mechanism which is actuated bythe safety cover of the die closing unit of an injection moldingmachine, arresting the movable die carrier member, whenever the safetycover is not in its closed position.

2. Description of the Prior Art

From the prior art in this field are known several safety coverinterlock devices, including one which is disclosed in my prior U.S.patent application, Ser. No. 170,920, now U.S. Pat. No. 4,340,346. Thisdevice features two arresting rods which are attached to the movable diecarrier frame so as to extend rearwardly through bores in a stationarycylinder head plate of the die closing unit. To the rear side of thiscylinder head plate are attached two cooperating arresting units, eachenclosing a pair of toothed arresting jaws which are movable into andout of engagement with annular teeth of the arresting rods.

The movements of the arresting jaws are obtained by means of controlrods which are attached to the safety cover of the die closing unit andextend likewise rearwardly through the cylinder head plate and itsattached arresting unit. Appropriate cam surfaces on the control rodscooperate with cam followers which forcibly separate the arresting jaws,when the safety cover is closed. Springs press the arresting jawsagainst the arresting rod, as soon as the safety cover is opened a shortdistance. The arresting jaws are arranged on opposite sides of eacharresting rod, being guided for straight-line movements by the walls ofthe arresting unit housing. The cam followers are similarly arranged onopposite sides of the control rod, each cam follower being connected toits associated arresting jaw by means of a transverse follower shaft anda pair of drive links.

One shortcoming which is common to this and all the other known priorart interlock devices is that the approach movement which produces thearresting engagement is not a positive movement but is produced bysprings or, in some cases, by gravity. It is therefore possible todefeat the interlock action of each one of these devices by blocking themechanism in the disengaged position.

Another shortcoming of the device just described relates to the factthat the latter provides an arresting action only against the closingmovement of the movable die carrier member, because of itssawtooth-shaped succession of annular grooves defining the abutmentflanks which provide the arresting action. This groove profile is onewhich combines planar abutment faces with relatively narrow peripheralshoulders. The latter have to be as narrow as possible, in order tominimize the possibility that the approaching arresting jaws, which havesimilar shoulders on their inner arcuate peripheries, butt against theshoulders of the arresting rod.

SUMMARY OF THE INVENTION

Underlying the present invention is the primary objective of improvingupon the above-mentioned mechanical interlock device in such a way thatan opening movement of the safety cover produces a forcible closingmovement on a pair of arresting jaws against an arresting rod, withoutthe possibility of a butting interaction between the jaws and the rod.

The present invention proposes to attain this objective by suggesting animproved mechanical safety cover interlock mechanism for the die closingunit of an injection molding machine, the mechanism featuring anarresting rod which has a series of annular arresting collars and movesthrough an arresting unit. The latter encloses two arresting jaws withmatching arcuate ridges which are closable against the arresting rod bymeans of a control rod and a pair of cooperating cam followers. Thearresting jaws are forcibly closed against the control rod, as soon asthe safety cover is moved from its closed position, the jaw openingmovement being produced by compression springs.

In a preferred embodiment of the invention, the arresting collars on thearresting rod have axially oppositely facing abutment flanks, and thearresting jaws have cross-sectionally similarly shaped arresting ridgeswith opposing abutment flanks engaging the flanks of the arresting rodcollars in the closed jaw position. The arresting jaws are enclosed andguided between two guide walls of the arresting unit housing whichtransmits the arresting forces to a stationary member of the die closingunit. The arresting action is the same against a die opening movement asagainst a die closing movement.

The present invention further suggests an arrangement of the arrestingrod collars and cooperating arresting jaw ridges in such a way thatforcible engagement of at least one arresting jaw can take place underany operational condition, the configuration being such that it isimpossible to obtain a butting action between arresting collars andarresting ridges on both arresting jaws. This is accomplished by anaxial offset of the arresting ridges of one jaw relative to those of theother jaw by one-half the axial distance between arresting ridges, incombination with an axial spacing of the arresting collars and arrestingridges to a pitch at which the collar distance is equal to or greaterthan twice the combined axial widths of the arresting collar and thearresting ridge.

By further arranging for the arresting jaws and the associated movementmechanism to be "floating", i.e. indeterminate in its position in thedirection of jaw movement relative to the arresting unit housing, it ispossible to always obtain engagement of at least one of the twoarresting jaws, if a butting condition is encountered on the otherarresting jaw. The closed position of the arresting jaws is preferablysuch that, in a butting condition between arresting collars and thearresting ridges of one jaw, the arresting ridges of the other jaws arefully engaged between the arresting rod collars.

The proposed floating arrangement of the arresting jaws and of theassociated control mechanism allows for a butting action on either ofthe two arresting jaws. In the event that no butting takes place, biassprings move the floating assembly to one side, so that full engagementof one jaw, rather than partial engagement of both jaws, is obtained.

In order to obtain the indeterminate condition of the arresting jawsduring their closing movement in conjunction with a positively operatingjaw drive mechanism, the invention further suggests a control rod which,while driving a pair of cam followers which are attached to thearresting jaws, is itself capable of shifting in a transverse direction,to accommodate the different positions of the closed arresting jawswhich may be necessary for various arresting rod positions. This isaccomplished by attaching one extremity of the control rod to its drivearm by means of a universal joint.

The novel arresting mechanism of the present invention offers the uniqueadvantage of providing a positive, tamper-proof jaw closing action,without the possibility of ever developing a jamming condition betweenthe arresting jaws and the arresting rod in any arresting rod position,including situations in which the arresting rod is in motion. Unlikearresting jaws which are closed by spring action and which,consequently, can easily be maintained in an open state to defeat thearresting mechanism, the suggested novel arresting mechanism with itspositive jaw closing action can only be defeated by dismantling itscontrol linkage.

BRIEF DESCRIPTION OF THE DRAWINGS

Further special features and advantages of the invention will becomeapparent from the description following below, when taken together withthe accompanying drawings which illustrate, by way of example, anembodiment of the invention which is represented in the various figuresas follows:

FIG. 1 shows, in a somewhat schematic elevational view, a die closingunit of an injection molding machine with a mechanical safety coverinterlock mechanism embodying the present invention, the safety coveritself being not shown;

FIG. 2 shows the die closing unit of FIG. 1 in an enlarged end view;

FIG. 3 shows the arresting unit of the safety cover interlock mechanismof FIG. 1 in an enlarged horizontal cross section taken along lineIII--III of FIG. 4, the mechanism being shown in its normally disengagedcondition;

FIG. 4 shows the arresting unit of FIG. 3 in an elevational crosssection taken along line IV--IV of FIG. 3;

FIGS. 5 and 6 are similar to FIGS. 3 and 4, showing the mechanism of thearresting unit in a first arresting configuation;

FIGS. 7 and 8 are likewise similar to FIGS. 3 and 4, showing themechanism of the arresting unit in a second arresting configuration;

FIG. 9 represents an enlarged portion of the die closing unit of FIG. 1,including a portion of the safety cover;

FIG. 10 is a rotated plan view corresponding to FIG. 9; and

FIG. 10a shows a further enlarged universal joint forming a part of themechanism of FIGS. 9 and 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The die closing unit of FIGS. 1 and 2, portions of which are also shownin FIGS. 9 and 10, is mounted on a reinforcement ledge 21a of a machinebase 21. It consists essentially of a stationary die carrier plate 10, amovable die carrier frame 11, and a stationary cylinder head plate 12,the plates 10 and 12 being attached to the ledge 21a and interconnectedby means of four tie rods 13 to form a rigid frame.

The movable die carrier frame 11 is guided on the tie rods 13 forlongitudinal opening and closing movements along the main axis a--a ofthe die closing unit. The four tie rods 13 include special clampingassemblies 17 on both extremities. The movable die carrier frame 11 isadditionally supported on the lateral ledges 21a of the machine base 21by means of adjustable slide shoes 22.

The die carrier frame 11 includes a die mounting wall 11a, a rearwardlyspaced pressure transfer wall 11b, and intermediate pressure transferribs 11c. On its die mounting wall 11a, it carries one of the two halvesof an injection molding die 41, the other die half being mounted on thestationary die carrier plate. Extending rearwardly from the stationarycylinder head plate 12 is a coaxially mounted power cylinder 16 whosepiston rod 19 is connected to the pressure transfer wall 11b of themovable die carrier frame 11.

The die area 40 is normally covered by a generally U-shaped safetycover, but this cover is not shown in FIG. 1. The rearward end portionof the cover panel 75 in its closed position is shown in FIGS. 9 and 10.

FIG. 1 also shows, as part of the present invention, an arresting rod 14which extends rearwardly from the pressure transfer wall 11b of themovable die carrier frame 11, in parallel alignment with the tie rods13. The arresting rod 14a has on its surface a series of regularlyspaced annular arresting collars 14a. It cooperates with an arrestingunit of which the housing 15 is mounted on the rear side of thestationary cylinder head plate 12, as can be seen in FIGS. 1 and 2. Thearresting rod 14 extends through both the plate 12 and the housing 15,moving freely in the axial direction, unless the arresting unit isactuated, in the manner described further below.

The arresting unit of the invention is shown in FIGS. 3-8 whichrepresent three different operational configurations, FIGS. 3 and 4showing the arresting unit in a disengaged or open configuration, andFIGS. 5 and 6 and FIGS. 7 and 8, respectively, showing the arrestingunit in two different actuated or closed configurations.

As can be seen in FIG. 3, the arresting unit has a generallyblock-shaped housing 15 consisting of two parallel transversely orientedguide walls 44, an intermediate end wall 47, and a U-shaped housingshell 46. The end wall 47 on one side of the housing 15 and two guidewall spacers 70 on the other side determine the axial distance betweenthe guide walls 44, in conjunction with four heavy housing bolts 45which clamp the housing assembly against the outer side of thestationary head plate 12 (FIGS. 1 and 9). The heads 45a of the housingbolts 45 are visible in FIG. 2. The housing shell 46 is attached to theguide walls 44 and to the end wall 47 by means of screws 48.

The two guide walls 44 of the housing 15, as well as the adjoiningcylinder head plate 12 (not shown in FIG. 3), have appropriate openings71 for the moving arresting rod 14. Inside the housing 15 are arranged,on opposite sides of the arresting rod 14, two arresting jaws 34 and 35which occupy the entire rectangular cross section of the housing cavity,thus being guided for opening and closing movements at right angles tothe arresting rod 14.

The arresting rod 14 carries a series of axially regularly spacedarresting collars 14a, the latter having a rectangular cross-sectionaloutline so as to present planar abutment flanks 14b in both axialdirections. The arresting jaws 34 and 35 have cross-sectionallysimilarly shaped arcuate arresting ridges 34a and 35a, respectively,with axially oppositely facing abutment flanks 34b and 35b,respectively. It follows that, when one or both of the two arrestingjaws 34 and 35 are engaged against the arresting rod 14, the latter ispositively blocked against axial advance or return, the blocking forcebeing transmitted by the engaged locking jaw to one of the guide walls44 and from there to the stationary cylinder head plate 12 (FIG. 1).

FIG. 3 shows an arrangement of the arresting rod collars 14a and thecooperating arresting jaw ridges 34a and 35a which is such that,regardless of the momentary axial position of the arresting rod 14,radial butting between the outer peripheral surfaces of the arrestingcollars 14a and the inner peripheral surfaces of the arresting ridges34a and 34b can never take place on both arresting jaws 34 and 35simultaneously. This is accomplished by a configuration which meets twospecific geometric conditions in terms of the axial distance, or pitch,of the arresting collars 14a of the arresting rod 14 and in terms of anaxial offset between the cooperating arresting collars and arrestingridges on opposite sides of the arresting rod 14.

The first condition requires that the axial pitch of the arrestingcollars 14a on the arresting rod 14 be equal or greater than twice thecombined axial widths of the arresting collars 14a and the arrestingridges 34a or 35a. Therefore, if the two axial widths are identical, theaxial pitch of the arresting collars and arresting ridges has to beequal or greater than four times that axial width.

The second condition requires an axial offset of one set of arrestingmembers by one-half this axial pitch. FIG. 3 shows the arresting jaws 34and 35 to have such an axial offset in their arresting ridges 34a and35a, respectively. Obviously, the same effect can also be achieved byarranging the axial offset on the arresting collars 14a of the arrestingrod 14. In this case, however, the annular grooves which form thearresting collars 14a of FIG. 3 would preferably be replaced by straighttangent grooves and the arresting ridges of the cooperating arrestingjaws would then likewise be straight, instead of arcuate.

The arresting jaws 34 and 35 are closed and opened by means of a controlrod 18 and a pair of cooperating cam followers 27 and 27a. The controlrod 18 extends approximately parallel to the arresting rod 14, reachingthrough an opening of the stationary cylinder head plate 12 (not shownin FIG. 3) and a bore 64 in the guide wall 44 of the housing 15. As willbe explained in more detail further below, the control rod 18 executesan actuation stroke by penetrating into the housing 15, when the safetycover of the die closing unit is moved from its closed position.

The control rod 18 has a rectangular cross section, with and end portion18a of reduced width and two transitional portions which form camsurfaces 18b on the narrow sides of the rectangular rod. Cooperatingwith these narrow sides and the cam portions 18b are two cam followers27 and 27a in the form of ball bearings which are rigidly connected tothe two arresting jaws 34 and 35, respectively.

FIG. 4 shows that the cam follower 27a is arranged inside a groove 50 ofthe proximate arresting jaw 35 and supported by a shaft 32a which isseated in the jaw 35. The cam follower 27 is supported by a similarshaft 32 which is supported on its extremities in a pair of drive links30 which, in turn, are attached to the distal arresting jaw 34 by meansof link pins 43. Appropriate grooves in the arresting jaw 43 positionthe drive links 30, and similar slightly larger grooves in the arrestingjaw 35 allow for a free relative movement between the jaw 35 and thedrive links 30.

The two shafts 32 and 32a which carry the cam followers 27 and 27a alsocarry two counter rollers 65 and 65a, likewise in the form of ballbearings, which are spaced from the cam followers 27 and 27a by means ofspacer washers 66. The counter rollers 65 and 65a ride on a counter rail49, thereby supporting the cam followers 27 and 27a against the axialforce component which results from a forcible advance of the control rod18 against the cam followers, during the actuation of the arrestingunit.

FIG. 4 further shows that the two arresting jaws 34 and 35 arespring-biased into the disengaged or open position by means of two pairsof jaw opening springs 51 which are arranged on opposite sides of thearresting rod 14. The springs 51 thus produce a contact pressure betweenthe cam followers 27 and 27a and the cam portions 18b of the control rod18. Two jaw biasing springs 38, their function to be described furtherbelow, are arranged between the housing end wall 47 and the distalarresting jaw 34.

As stated earlier, the arresting unit is actuated, whenever the safetycover of the die closing unit is moved from its closed position. Thismovement is transmitted to the control rod 18 by means of a transmissionlinkage, described further below, producing an actuation stroke on thecontrol rod 18 which advances the latter from the position shown in FIG.3 to the position shown in FIG. 5. In so doing, the control rod 18forces the cam followers 27 and 27a to ride up on its cam portions 18b,thereby approaching the connected arresting jaws 34 and 35,respectively, towards the arresting rod 14.

As can be seen by comparing FIGS. 5 with 7, one or the other of thearresting jaws 34 and 35 may develop a butting condition between itsarresting ridges 34a or 35a, respectively, and the rim of the arrestingcollars 14a of rod 14. However, the earlier-described arresting geometryassures that, for all possible positions of the arresting rod 14, onlyone of the two arresting jaws can develop such a butting condition.

The mechanism of the invention tolerates unilateral butting on eitherside of the arresting rod 14 by providing for the two arresting jaws 34and 35, their connected cam followers 27 and 27a, respectively, and thecontrol rod 18 to be "floating", i.e. indeterminate in position,relative to the arresting rod 14, during the closing movement of thearresting jaws 34 and 35.

For this purpose, the control rod 18 includes a pivot connection withthe transmission linkage, so that at least that portion of the controlrod 18 which reaches into the arresting unit housing 15 is free to movein the direction of cam follower movement. Thus, when one cam followeris blocked in position by a butting condition between its arresting jawand an arresting rod collar, the other cam follower and its arrestingjaw execute a movement corresponding to the combined rise of both camportions 18b of the control rod 18.

FIGS. 5 and 6, representing a butting condition between the distalarresting jaw 34 and the arresting rod 14, show the cam follower 27blocked in place, while the opposite cam follower 27a has produced acomplete engagement between the arresting jaw 35 and the arresting rod14. Conversely, FIGS. 7 and 8 show the presence of a butting conditionbetween the near arresting jaw 35 and the arresting rod 14, as a resultof which the cam follower 27a is blocked in place, while the oppositecam follower 27 and its connected distal arresting jaw 34 have beenmoved for a full engagement between the jaw 34 and the arresting rod 14.

A third arresting configuration would be present, if the arresting rod14 is in such a position that neither the arresting ridges 34a of jaw 34nor the arresting ridges 35a of jaw 35 encounter an arresting collar14a, so that both arresting jaws 34 and 35 are free to advance againstthe arresting rod 14. This configuration is not shown in the drawings,and, because it can produce only a partial engagement of both arrestingjaws, it is not desirable and therefore intentionally prevented in themechanism of the present invention. Such a result is obtained by meansof the earlier-mentioned jaw biasing springs 38 which, in effect,produce the same jaw closing condition as would be produced by a buttingcondition between the arresting ridges 35a of the near arresting jaw 35and the arresting rod collars 14a.

The action of the jaw biasing springs 38 makes the configuration ofFIGS. 7 and 8 the normal arresting position for the interlock mechanismof the invention. Consequently, the position of the arresting jaws 34and 35 in FIG. 5 will tend to switch to the position shown in FIG. 7, ifthe arresting rod 14 is moved axially so that the arresting ridges 34aof jaw 34 are no longer butting against the arresting collars 14a of rod14. On the other hand, the jaw biasing springs 38 are considerablyweaker than the jaw opening springs 51, so that, even in the openposition of the arresting jaws as shown in FIG. 4, for example, thedistal arresting jaw 34 is safely held out of engagement with thearresting rod 14.

A closing motion of the safety cover produces a retraction of thecontrol rod 18 from the position of FIG. 5 or FIG. 7 to the position ofFIG. 3, allowing the cam followers 27 and 27a to execute an approachmovement under the bias of the jaw opening springs 51. The open positionof the arresting jaws 34 and 35 is determined by the width of the endportion 18a of the control rod 18 and by an abutment between an end face49a of the counter rail 49 and the bottom surface 50a of the groove 50in the arresting jaw 35. The jaw biasing springs 38 produce the abutmentforce.

The transverse position of the camming portion of the control rod 18 isdetermined by the positions of the arresting jaws 34 and 35 and theircam followers 27 and 27a, respectively. To guide the arresting rod 18 inthe vertical sense, i.e. in the plane e--e of FIG. 4, the guide wallopening 64 may have an appropriate transverse guide insert (not shown),or the opening itself may be of rectangular rather than round shape.

FIGS. 9 and 10 show an example of a control linkage which transmits anactuating force to the control rod 18 of the arresting unit, as soon asthe safety cover of the die closing unit is moved away from its closedposition. FIG. 9 shows a corner portion of a closed safety cover panel75 which is guided for axial opening and closing movements in relationto a stationary frame member 62. An opening movement of the cover panel75 would be a movement to the left side of FIG. 9.

In the stationary frame member 62 is journalled a transversely orientedcrank pin 61 which carries a control arm 58 and an aligned crank 57 onopposite ends. The crank 57, in turn, carries a cam follower 76 whichrides on a ramp 75a of the cover panel 75. In the closed position of thecover panel 75, shown in FIG. 9, the cam follower 76 engages an inclinedend portion of the ramp 75a which allows the crank 57 and the controlarm 58 to assume substantially vertical orientations.

As soon as the cover panel 75 is opened, i.e. moved to the left in FIG.9, the cam follower 76 swings to the left, until it comes to rest on thehorizontal portion of the ramp 75a. This movement produces acorresponding displacement of the control arm 58 to which is connected arearwardly extending transmission rod 59. The opposite end of thetransmission rod 59 is connected to the end of a pivot arm 60, on apivot shaft 55, which is rotatably mounted on the cylinder head plate 12by means of pivot brackets 63 (FIG. 10). The pivot shaft 55 carries apivot arm 60 whose distal extremity is connected to the control rod 18by means of a universal link 54.

The universal link 54 which transmits the rearward motion of thetransmission rod 59 to the control rod 18 is shown in an enlargedperspective view in FIG. 10a. A vertical slot 78 in one end portion ofthe link 54 accommodates the upper extremity of a drive arm 69 which,like the pivot arm 60, is fixedly attached to the pivot shaft 55. A pin56 in the horizontal bore 68 provides the drive connection. A horizontalslot 77 in the opposite end portion of the universal link 54accommodates an extremity of the control rod 18, and a vertical pin 53(FIG. 3) in the vertical bore 67 constitutes the pivot about which thecontrol rod 18 executes its transverse "floating" movements.

It should be understood, of course, that the foregoing disclosuredescribes only a preferred embodiment of the invention and that it isintended to cover all changes and modifications of this example of theinvention which fall within the scope of the appended claims.

I claim the following:
 1. In a die closing unit for an injection moldingmachine which comprises a stationary die half mounted on a stationarydie carrier member, a movable die half mounted on a movable die carriermember, a plurality of parallel tie rods guiding the movable die carriermember and defining the longitudinal axis of the die closing unit, asecond stationary member located axially outside the area of theinjection molding die, behind the movable die carrier member, and anaxially openable and closable safety cover preventing access to at leasta portion of the area of the injection molding die in its closedposition; in said die closing unit, a mechanical safety cover interlockmechanism comprising in combination:an arresting rod attached to themovable die carrier member and extending axially rearwardly towards thesecond stationary member, in parallel alignment with the tie rods, saidstationary member having an opening for the axial movement of thearresting rod therethrough; an arresting unit mounted on the secondstationary member in alignment with the arresting rod and surroundingthe latter; and a control linkage operatively connecting the safetycover to the arresting unit in such a way that a displacement of thesafety cover from its closed position actuates the arresting unit in thearresting sense and the complete closing of the safety cover actuatesthe arresting unit in the releasing sense; and wherein the arresting rodhas, on at least that length portion which moves through the arrestingunit, a regularly spaced succession of grooves and intermediatearresting collars which occupy at least two diametrally opposite sidesof the arresting rod, the arresting collars having at least one of theirtwo axially opposite flanks oriented transversely to the arresting rodaxis, so as to serve as an abutment flank; the arresting unit includestwo arresting jaws arranged on diametrally opposite sides of thearresting rod so as to face the arresting collars of the latter, thearresting jaws being guided for movement into and out of engagement withthe arresting rod, each arresting jaw having at least one arrestingridge with at least one of its two axially opposite flanks oriented tocooperate with the abutment flank of an arresting rod collar to producean arresting action on the arresting rod and connected movable diecarrier member; the arresting ridges of the two arresting jaws and thearresting collars on opposite sides of the arresting rod are axiallystaggered in such a way that, when the arresting rod is positioned tocreate a butting alignment between an arresting rod collar and anarresting jaw ridge on one side, the ridge of the other arresting jaw ispositioned axially half-way between two successive rod collars on theother side; the axial spacing of the arresting collars on the arrestingrod is not less than twice the combined axial widths of an arrestingcollar and an arresting ridge, so that a simultaneous buttingimpingement between arresting collars and arresting ridges on both sidesof the arresting rod during the engagement approach of the arrestingjaws is made impossible for all arresting rod positions; the controllinkage includes means for actuating the arresting unit by forciblyclosing the arresting jaws against each other to obtain a radialengagement between abutment flanks of the arresting rod collars and thearresting jaw ridges, the jaw closing force being derived from thesafety cover displacement; said arresting jaw closing means engages thearresting jaws in such a way that the jaw closing force acts on the twoarresting jaws without a force reaction from a stationary part of thearresting unit, so that the position of the arresting jaws during theirclosing movement, while being determinate relative to each other, isindeterminate relative to the arresting rod within the guided mobilityof the arresting jaws, and the closed distance of the arresting jaws issuch that the arresting ridge of one jaw remains out of engagement withthe arresting rod, when the arresting ridge of the other jaw is fullyengaged in an arresting rod groove.
 2. A safety cover interlockmechanism as defined in claim 1, whereinthe arresting collars andintermediate grooves of the arresting rod are annular collars andannular grooves, the arresting collars having both axial flanks orientedtransversely to the arresting rod axis, to serve as opposite abutmentflanks; and the two arresting jaws each have a plurality of concavelyarcuate arresting ridges of the same axial spacing as the arresting rodcollars, the arresting ridges having their opposite axial flanksoriented to cooperate with opposing abutment flanks of the arresting rodcollars, so as to produce arresting actions in both axial directions;and the arresting ridges of one arresting jaw are axially offset inrelation to the arresting ridges of the other jaw by one-half thedistance of said spacing.
 3. A safety cover interlock mechanism asdefined in claim 1 or claim 2, whereinthe arresting unit furtherincludes jaw opening spring means urging the arresting jaws away fromeach other, into an open position, in opposition to the arresting jawclosing means; and the jaw opening spring means, like said jaw closingmeans, act on the two arresting jaws without a force reaction from astationary part of the arresting unit.
 4. A safety cover interlockmechanism as defined in claim 3, whereinthe arresting unit furtherincludes bias spring means biasing one arresting jaw in relation to astationary part of the arresting unit in the direction of the guidedmobility of the jaw, thereby applying said bias to both arresting jawsin those jaw positions that would otherwise be indeterminate; and theforce exerted on said arresting jaw by the bias spring means is lessthan the force exerted on it by the jaw opening spring means.
 5. Asafety cover interlock mechanism as defined in claim 3, whereinthecontrol linkage further includes a pivot shaft which is journalled onthe second stationary member and oriented perpendicularly to the controlrod, a pivot arm attached to the pivot shaft, a transmission rod linkingthe distal extremity of the pivot arm to the safety cover, and a drivearm attached to the pivot shaft in an orientation which is substantiallytransverse to the control rod; and the distal extremity of the drive armis connected to an extremity of the control rod by means of a universaljoint.
 6. A safety cover interlock mechanism as defined in claim 3,whereinthe arresting jaw closing means of the control linkage includes alongitudinally movable control rod extending perpendicularly to themovement direction of the arresting jaws and deriving its movements fromthe safety cover movements, the control rod having cam portions whichface in opposite radial directions; each of the two arresting jaws hasconnected to it a cam follower, the two cam followers cooperating withthe two cam portions of the control rod in such a way that alongitudinal movement of the latter displaces the cam followers inopposite directions, to forcibly close the arresting jaw, and anopposite longitudinal movement of the control rod allows the arrestingjaws to be opened; and the control rod also has a lateral mobility inthe direction of displacement of the cam followers and arresting jaws,in at least that portion of the control rod which cooperates with thecam follower, thereby making possible said indeterminate state of thearresting jaws during their closing movement.
 7. A safety coverinterlock mechanism as defined in claim 6, whereinthe control rodextends substantially parallel to the arresting rod, alongside one ofthe two arresting jaws, moving in the same direction as the safety coverto which it is connected; one cam follower is rotatably supported on theproximate arresting jaw, and the other cam follower is rotatablysupported on an extension of the distal arresting jaw; and the arrestingunit further includes an arresting unit housing enclosing the arrestingjaws and their cam followers, the housing having two walls which extendperpendicularly to the arresting rod axis and serve as guide walls forthe moving arresting jaws and as arresting force transmitting walls,when the jaws engage the moving arresting rod.
 8. A safety coverinterlock mechanism as defined in claim 7, whereinthe cam followers aresupported on pins which also carry counter rollers; and the arrestingunit housing further includes a counter roller supporting surface facingagainst the jaw closing movement of the control rod.